I don't know of any particular sites that list the little reminders of past attractions, but:
--The first car on the right as you enter the Test Track queue is from World of Motion.
--The planter between Innoventions East and Mission: Space is in the shape of the old Horizons building.
--The old Horizons logo is visible in two places in Mission: Space, one at the center of the gravity wheel in the queue, and another on the gift shop's counter.
--The Jeopardy announcer in Ellen's Energy Adventure makes a reference to "Energy, you make the world go 'round," one of the songs in the previous incarnation of the attraction.
--Dreamfinder is memorialized in the current version of Journey Into Imagination with a door marked "Dean Finder."
--Some of the prisoner teleportation pods in Stitch's Great Escape have old XS Tech logos from Alien Encounter. (and Alien Encounter, itself, referenced 'Mission to Mars,' the previous occupant of the space, in its pre-show)
--The Tomorrowland area music includes "Now is the Time" from the original WDW version of Carousel of Progress, "If You Had Wings" from the attraction of the same name and "Miracles from Molecules" from Adventure through InnerSpace (an extinct Disneyland attraction).
I know there are a ton more, but that's all I can think of at the moment.
Edit: also, the full Innoventions music loop, which is only heard in a small area of MouseGear, includes lots of old Epcot music, such as "Listen to the Land," "Energy, you make the world go 'Round" and "New Horizons." The Land's area music also still features "Listen to the Land."
Further edit: One of the old Dream Mobile props from the original Journey Into Imagination sits above the CD and DVD section of MouseGear
--The Living with the Land fastpass machines still bear The Land's logo.
--Until fairly recently, Figment's Place featured the old Imagination logo.